in_tcp: skip emitting an empty event stream when a read has no complete message #5602

Is your feature request related to a problem? Please describe.

Without <extract>, in_tcp calls router.emit_stream(@tag, es) after every read. It does this even when the read did not complete any message and es is empty. A long message arrives in many reads, so the router, the filters and the output are called many times with an empty stream. The output's emit_count counts these calls too.

With the configuration below, I sent one 1 MiB line in 64 writes of 16 KiB:

ruby -rsocket -e 's = TCPSocket.new("127.0.0.1", 5170); 64.times { s.write("x" * 16384); sleep 0.02 }; s.write("\n"); sleep 1'

monitor_agent then shows:

{"plugin_id":"in_tcp","emit_records":1}
{"plugin_id":"out_null","emit_records":1,"emit_count":66}

<source>
  @type monitor_agent
  bind 127.0.0.1
  port 24220
</source>

<source>
  @type tcp
  @id in_tcp
  tag test
  port 5170
  bind 127.0.0.1
  <parse>
    @type none
  </parse>
</source>

<match test>
  @type null
  @id out_null
</match>

Describe the solution you'd like

Call router.emit_stream only when es is not empty. The <extract> code path already emits nothing for such a read.

Describe alternatives you've considered

Keep the current behavior. The empty streams do not create buffer chunks, because Buffer#write_once returns early for empty data. The cost is the extra calls and the larger emit_count.

Additional context

Tested with Fluentd 1.20.1 (master at 1d6b112) and Ruby 4.0.9 on Linux.